September 29, 2010

It's all about size: Martian dust devils

I use this image as my desktop, so each day I gaze at it in wonder. It’s
natural art and design, somehow mysterious, somehow sensual. This
extraordinary – and much-publicised – image is from Mars, captured by the high
resolution orbiter, and reveals the tracks of dust devils, whirling miniature
tornado dervishes of wind engraving their way over the dunes. But exactly
how these dark tracks form has been something of a mystery: is it to do
with composition, that light dust is removed to reveal sand of darker materials
beneath?

Ironically, more observations had been made of this phenomenon on Mars than
on Earth – orbiters and rovers have gathered a gallery of images and movies
of dust devils on the Red Planet:

Earlier this year, Dennis Reiss, a geographer at Westfälische Wilhelms
Universität Münster in Germany, together with his colleagues, reported on
their earth-bound fieldwork in the Turpan Desert of northwestern China. Dust
devil tracks are small and ephemeral things here, whereas the Martian designs
are larger and last far longer. But the workers in China were able to observe
dust devils at work and examine the character of the dark trails left behind.
What they showed was that the appearance of the tracks was dependent on the size
of the grains. The sand grains outside the tracks were coated with fine dust
that brightly reflects the light. Within the tracks the blast of the passing
dust devil blew the sand along, tumbling and saltating, with
the result that the dust was picked up by the vortex and clean sand grains were
left behind. Smaller grains appear lighter than coarse ones – they reflect more
light and their albedo is higher – and so the tracks appear darker than
the surrounding sand.

So far, so good. But there are examples, as recently
reported by Mary Pendleton Hoffer and Ronald Greeley at Arizona State
University, of dust devil tracks on Mars that are lighter than their
surroundings:

Reiss and his colleagues have now
reported on light-coloured tracks from the Turpan Desert. These appeared in
sand that had been darkened by a brief rain shower the previous night. But
invoking rain on Mars is clearly a non-starter, so what is going on? Well, it
turned out that the sand within the light tracks was no different from usual –
millimetre-sized grains cleansed of finer material. But, the darkening
of the surrounding sands was a result of the grains being cemented together by
the rain into clumps up to a centimetre across. Big clumps = low albedo = dark
appearance. The dust devil had broken these up, brightening the sand left behind
in its track.

OK, but there’s still no rain on Mars. However, there are forces that can
clump sand grains together – electrostatic forces. Such clumping has been
observed by the field geologists on Mars, the indomitable rovers.

In 1979, Greeley conducted lab experiments showing that charges build up on
dry, wind-blown sand particles in a similar manner to the way charge builds up
on a balloon when you rub it on your hair. Just like the charged-up balloon can
stick to the wall, charged sand grains pull together to form delicate, “popcorn
ball” aggregates.

“The destruction of aggregates on Mars would lead also to bright dust devil
tracks,” Reiss said.

Greeley thinks the idea makes sense. “This is a plausible model for the
formation of the bright tracks on Mars,” he said. “This is a very nice study, a
very nice result.”

So, through questions raised on Mars we are better understanding processes on
our own planet. The wonder of Martian calligraphy is, however, by no means
diminished.

[See the amazing gallery of Mars high resolution images at the HiRISE site. Images courtesy of
NASA/JPL/University of Arizona HiRISE]

"Fortunately" has nothing to do with it - it's a simple matter of rationality.....

The designs of nature are fundamentally sensitive to scale and perspective - the Nazca lines or the Martian dust devil tracks look completely different from the ground and from some random altitude. And I would include the creators of the Nazca lines in the spectrum of the creative agents of nature.